There are different systems in the market for detecting presence or absence of objects. For example, some of these systems are for security use and make use of motion detection devices. Some other systems make use of mechanical sensing techniques. In these other systems, pressure sensors or inductive-capacitive proximity sensor would be used. While these sensors are effective to some extent, they suffer from a number of limitations. For example, they tend to have shorter life span. Further, pressure sensors cannot distinguish between whether an object detected is a person or merely a non-living object. For inductive-capacitive sensors, they are typically (electrical) energy consuming and would complicate their applications. Yet further, many of the existing sensing technologies may not be suitable for use in busy or crowded areas. For inductive-capacitive proximity sensors, the use of electrodes is typically required. However, if the inductive-capacitive proximity sensor is accidentally exposed or earthed, e.g. when water is present, the detection will become inaccurate. Also, the electromagnetic field generated by such sensor may be interfered by external field surrounding it.
The present invention seeks to address the above issues or at least to provide a useful alternative to the public.